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  1. #1
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    Garrett: Wheel trim and housing A/R sizing tech articles

    Hey guys, here a couple more articles from Garrett and they are closely related so i'll just post them together. All credit goes to Garrett Turbochargers.


    Wheel Trim
    Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say "I have a GT2871R 56 Trim turbocharger". What is 'Trim?' Trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio.
    * The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.
    Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter (see Figure 1.) Click here to enlarge

    Figure 1. Illustration of the inducer and exducer diameter of compressor and turbine wheels
    Example #1:
    GT2871R turbocharger (Garrett part number 743347-2) has a compressor wheel with the below dimensions. What is the trim of the compressor wheel?
    Inducer diameter = 53.1mm
    Exducer diameter = 71.0mm


    Click here to enlarge
    Example #2:
    GT2871R turbocharger (part # 743347-1) has a compressor wheel with an exducer diameter of 71.0mm and a trim of 48. What is the inducer diameter of the compressor wheel?
    Exducer diameter = 71.0mm
    Trim = 48


    Click here to enlarge
    The trim of a wheel, whether compressor or turbine, affects performance by shifting the airflow capacity. All other factors held constant, a higher trim wheel will flow more than a smaller trim wheel. However, it is important to note that very often all other factors are not held constant. So just because a wheel is a larger trim does not necessarily mean that it will flow more.


    Turbocharger A/R

    A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as:
    the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area (see Figure 2.).

    Click here to enlargeFigure 2. Illustration of compressor housing showing

    A/R characteristic
    The A/R parameter has different effects on the compressor and turbine performance, as outlined below.

    Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.

    Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.
    Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.
    When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.

    Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.

    Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:

    1. Engine #1 has turbine housing with an A/R of 0.63
    2. Engine #2 has a turbine housing with an A/R of 1.06.

    What can we infer about the intended use and the turbocharger matching for each engine?

    Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high back pressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.

    Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 since Engine #2 will be operating at high engine speeds most of the time.
    Click here to enlarge
    MOTIV750, MOTIV P-1000 PI, MOTIV/FUEL-IT! low pressure fuel system, AEM EMS/COBB AP, Aquamist HFS-3, ETS FMIC, SPEC stage 3+ clutch/SS flywheel, BC Racing coilovers and VMR wheels wrapped in Hankook RS3s.

  2. #2
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    Good thread!

  3. #3
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    This definately helped me as I am a newb to all aftermarket turbos. I had to learn fast and this was a good read. thanks.
    2011 335is DCT, JB4 + MHD BEF, stage 2 LPFP, e50 + 50/50 meth, FBO, MT ET Streets when needed


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